The present invention relates to liquid coating compositions which have a low solvent content or are solvent-free. They optionally contain conventional auxiliary agents and comprise a mixture of aminoplasts and polyesters.
Such ecologically acceptable coating compositions have been known for several years (U.S. Pat. No. 3,852,375; DAS 2,019,282). Also conventional are coating compositions which contain polyesters, the acid component of which is formed entirely or partially from hexahydroterephthalic acid units (German Pat. Nos. 1,644,766 corresponding to U.S. Pat. No. 3,553,284; 2,343,436 corresponding to British Pat. No. 1,471,779; 2,457,775 corresponding to U.S. application Ser. No. 686,483 filed May 14, 1976, now allowed; and DOS 2,437,217. German Pat. No. 1,644,766 expressly refrains from claiming low-solvent or solvent-free coating compositions. German Pat. No. 2,343,436 is limited with respect to the acids to be used in addition to hexadydroterephthalic acid, to aromatic dicarboxylic acids having carboxy groups in the 1,2- and/or 1,3-positions. German Pat. No. 2,457,775 also mentions only these isomers (see column 5, line 20).
As thorough investigations have shown, the coating compositions of the prior art possess a decisive disadvantage. In order to obtain coatings satisfying stringent requirements under practical testing conditions, baking temperatures of greater than 130.degree. C., in most cases even greater than 150.degree. C., (DOS 2,437,217, Example 3) heretofore had to be employed. To save energy costs, it is desirable to lower the baking temperatures, of course, while maintaining the satisfactory final properties. In processing the products of the prior art, baking temperatures of less than 150.degree. C. or less than 130.degree. C. can be utilized, but in such cases, it is necessary to substantially prolong the baking times so that the curing process loses its economic advantage and, moreover, no longer meets practical requirements. Furthermore, larger amounts of catalysts do not aid in achieving the desired result, since they impair the properties of the final coatings, such as, for example, the gloss.
DOS 2,437,217 describes coating compositions which, in accordance with the examples, can be baked at 120.degree. C. within a period of time satisfying practical requirements and produce satisfactory coatings. Experiments by the inventors, however, have shown that the coating compositions described therein, in contrast to the other products of the aforementioned prior art references, cannot be baked, for example, at 170.degree.-180.degree. C. (which is a temperature range customary for use in certain manufacturing processes), without the occurrence of defects in the coating. Due to such deficiencies produced by exposure to higher temperatures, the binders of DOS 2,437,217 are extremely sensitive to the increased thermal loads which occur, for example, when a conveyor belt comes to a standstill during series-enameling operations. Furthermore, these coating compositions do not exhibit sufficient stability during the baking step, as do, for example, the coating compositions of German Pat. No. 2,457,775.
Other relevant liquid coating compositions are disclosed in U.S. Pat. Nos. 3,553,284, 3,668,275, 3,668,276, 3,668,277, 3,678,128, 3,691,258 and 3,819,757, the latter of which discloses the use of a 1,4 dicarboxylic aromatic acid (terephthalic) as an acid component in a system significantly different from that of this invention.